1. Field of the Invention
The invention relates to thin films comprising mesoporous transition metal oxide materials. The invention further relates to a method of producing mesoporous transition metal oxide thin films via pulsed laser ablation of appropriate mesoporous molecular sieve targets. The invention also relates to the use of mesoporous transition metal oxide thin films produced by the method of the invention to manufacture chemical sensors and electrochromic devices.
2. Description of Related Art
Two classes of materials with wide ranging uses as heterogeneous catalysts, adsorption media, and as components of chemical sensors and electrochromic devices, are microporous (pore diameter less than 20 xc3x85) and mesoporous (pore diameter 20 xc3x85-500 xc3x85) inorganic solids. The utility of these materials is a consequence of their chemical structures, which allow guest molecules access to large internal void surfaces and cavities, thereby enhancing the catalytic activity and adsorptive capacity of these materials.
Typical of the microporous materials are the aluminosilicate molecular sieves known as zeolites. In zeolites, the micropores form regular arrays of uniformly-sized channels. Zeolites can act as hosts to ionic and neutral molecular guest species. However, the usefulness of sensors and devices fabricated from zeolites and other microporous materials is generally limited to those applications where the guest or analyte molecules have sufficiently small kinetic diameters to pass through the narrow microporous void openings.
Mesoporous materials offer the advantage of larger pore sizes, making them compatible with applications such as separation or sensing of relatively large organic molecules. Typical of the mesoporous materials are amorphous or polycrystalline solids such as pillared clays and silicates. Unfortunately, the pores in these materials are often irregularly spaced and broadly distributed in size, making them ill-suited for chemical separations, sensing and other device-oriented applications.
Considerable synthetic effort has therefore been devoted to developing molecular sieve frameworks with pore diameters within the mesoporous range, and a series of mesoporous molecular sieves having a hexagonal array of uniform mesopores has recently been developed (Beck et al., 1992). These materials, designated MCM-41, are of great interest because their large and uniform pore sizes allow otherwise sterically hindered molecules facile diffusion to internal active sites. However, the MCM-41 series of molecular sieves are silicate and aluminosilicate materials, and their aluminum and silicon centers do not have variable oxidation states, thus precluding the use of these materials in display applications and related electrochromic devices.
A method of precisely controlling deposition of a well-adhered mesoporous transition metal oxide thin film having redox active metal centers would be beneficial in extending the range of thin film materials available for use in applications such as electrochromic devices, chemical separations, and chemical sensing.
The present invention provides novel mesoporous transition metal oxide thin films, as well as processes for producing these thin films. The present invention further provides methods of fabricating useful chemical sensors and electrochromic devices from the thin films of the invention.
In forming the films of the present invention, a substrate is placed on a variable temperature substrate holder within a controlled-atmosphere chamber. The substrate is usually, though not always, heated in order to facilitate bond making between reactive molecular species produced during a laser ablation process. A pulsed laser beam is then directed into the controlled-atmosphere chamber and focused such that it impinges on a target of mesoporous transition metal oxide molecular sieve material, ablating the target. During the laser ablation process, a continuous, uniform film is deposited on the surface of the substrate. The film thickness is controlled primarily by adjusting the duration of the laser ablation/deposition process. Depending on the thickness of film desired, the time of deposition can range from a few seconds to over an hour.
In general, the thickness of the films will be dictated by their intended use. For example, electrochromic applications may require an extremely thin transparent film of about 50 nm thickness or less, whereas chemical sensing applications may require a somewhat thicker film in the range of about 200 nm to about 300 nm thickness.
The thin films of the present invention possess a mesoporous structure which can be further enhanced by means of a hydrothermal treatment. If necessary, the thickness of the thin films can actually be increased by means of the hydrothermal treatment step, as explained hereinbelow. The thin films can also be treated with an acid wash or other means of removing the templating agent originally used in the synthesis of the mesoporous target material.
The advantages of this invention over current technology are several. For example, the mesoporous transition metal oxide thin films have metal centers with variable oxidation states, allowing the fine tuning of catalytic, electronic and magnetic properties of these materials. Furthermore, the thin films of the present invention are deposited from a target of the desired mesoporous material using pulsed laser ablation, allowing more precise control of deposition of well-adhered thin films as compared with methods involving growth from solution at a substrate/solution interface. Also, the thin films of this invention may be deposited onto flexible polymer substrates, in addition to conventional metal, ceramic or glass substrates, thus decreasing the total weight of sensor or display devices manufactured therefrom.
In particular, the present invention provides thin films formed from mesoporous oxides of niobium, titanium, tantalum, zirconium, cerium, tungsten, molybdenum, iron, lead, and any other mesoporous oxides of transition metals. These mesoporous oxides of transition metals include, but are not limited to, Nb-TMS1, Ti-TMS1, Ta-TMS1, Zr-TMS1, Ce-TMS1, and related mesoporous oxides of tungsten, molybdenum, iron and lead.
Thin films, as used herein, are films that measure between about 10 nm and 100 xcexcm in thickness. In one preferred embodiment, the thin films may measure between about 200 nm and about 300 nm in thickness. In an alternative embodiment, the thin F films may measure between about 10 nm and about 50 nm in thickness. The appropriate thickness of the thin film will vary according its intended use.
For example, it may be desirable to employ a thin film of a mesoporous transition metal oxide about 50 nm in thickness in electrochromic applications where the thin film should be transparent in its colorless state. In another application, it may be desirable to employ a thin film of a mesoporous transition metal oxide about 250 nm in thickness as the dielectric phase in a capacitive-type chemical sensor. In this latter application, a fine balance exists between films that are too thin, such that electrical breakdown occurs, and films that are too thick, such that the change in dielectric properties is too small to be accurately measured.
The substrates upon which the thin films may be deposited include, but are not limited to, titanium nitride-coated silicon, indium-doped tin oxide-coated glass, indium-doped tin oxide-coated polyester, and other inorganic supports such as alumina. The appropriate choice of substrate will vary according the intended use of the thin film.
The inventors contemplate that the mesoporous transition metal oxide thin films of the present invention may be employed as the dielectric phase of capacitive-type chemical sensors. These capacitive-type sensors are expected to detect adsorbed volatile compounds via specific interactions with the large surface area presented by the mesoporous films. For example, the response of capacitive-type chemical sensors can be measured by an element which detects a change in the dielectric properties of the thin film upon adsorption of analyte molecules. This information is then processed by a transducer into a readable form such as a change in capacitance. It will be appreciated by those skilled in the art that other types of chemical sensors may be fabricated using the thin films of the present invention, such as those responsive to changes in resistance, impedance, weight, and other electrochemical or optical properties. Sensors responsive to weight changes include, but are not limited to, surface accoustic wave (SAW) and quartz crystal mass (QCM) monitor-type devices.
The inventors further contemplate that the mesoporous transition metal oxide thin films of the present invention may be employed as components of electrochromic devices in which redox reactions involving the metal centers of these thin films induce a color change in the film. Such electrochromic devices may have utility in applications including, but not limited to, electrochromic displays and mirrors, smart windows, and active camouflage.
For example, electrochromic mirrors in automobiles illustrate one possible application of electrochromic devices. At night, the lights of following vehicles can cause a dazzling reflection from the driver""s rearview mirror. This dazzling reflection can be prevented by use of an optically absorbing electrochromic film covering the reflecting surface. When the film is in its colorless state, the mirror functions normally. When the film is in its colored state, only a moderate amount of light is reflected. Smart windows illustrate another possible application of electrochromic devices. An entire window is coated with an electrochromic film, and this window may be darkened to reduce the flux of light or heat into, for example, a room, office or automobile. Yet another possible application of electrochromic devices is active camouflage, in which highly reflective surfaces such as windshields on, for example, aircraft, land-going vehicles, or sea-faring vessels are coated with an electrochromic film which can be darkened to decrease visibility of the aircraft, vehicle, or vessel.
Advantageously, the electrochromic devices of the present invention are expected to exhibit faster response times as compared with traditional dense phase metal oxide-based electrochromic devices. This is because the response time required for the electrochromic device to change between colored and colorless states depends on diffusion of charged species into or out of the film. The mesoporous structure of the films of the present invention are expected to allow an increase in the diffusion rate of charged species into and out of the film, compared to a dense phase metal oxide film.
In forming the mesoporous transition metal oxide thin films of the present invention, a laser ablation film deposition apparatus is used. Such an apparatus usually consists of a pulsed laser and a controlled-atmosphere chamber. The pulsed laser typically emits radiation pulses of about 14 ns duration at about 248 nm wavelength, and the repetition rate of laser pulses is typically about 10 Hz. However, the inventors contemplate that any laser which is sufficiently energetic to ablate a mesoporous transition metal oxide target may be used. The laser beam is directed into the controlled-atmosphere chamber, usually by means of a rastering mirror, and focused onto a mesoporous transition metal oxide molecular sieve target at an angle of about 45xc2x0.
The pressure in the controlled-atmosphere chamber is conveniently varied using a needle valve, or similar device for controlling or metering gas, and an oxygen source. The pressure in the controlled-atmosphere chamber may be varied between about 0.01 mTorr and about 600 mTorr, preferably between about 150 mTorr and about 300 mTorr.
By xe2x80x9caboutxe2x80x9d is meant xe2x80x9capproximatelyxe2x80x9d or xe2x80x9cin the vicinity of.xe2x80x9d For example, the phrase xe2x80x9cabout 150 mTorrxe2x80x9d may mean 151, 152, 153, 154 mTorr, etc., and fractional values therebetween, and it may also mean 149, 148, 147, 146, 145 mTorr, etc., and fractional values therebetween.
The use of a gas in the ablation process mainly serves to compensate for loss of constituent oxygen in the metal oxide. In the absence of an oxygen environment, laser deposited metal oxide films tend to be deficient in oxygen. In theory, any gas that can provide oxygen atoms, for example ozone, may be useful in this regard.
A variable temperature substrate holder is located below a target holder at a distance of about 2 cm to about 5 cm. A self-supporting target is made by pressing approximately 1.0 g of as-synthesized (containing organic template) mesoporous transition metal oxide compound in a die. A target formed in this manner will typically have a diameter of about 1 inch and a thickness of about xe2x85x9 inch.
In forming the films of the present invention, a desired substrate is placed on the variable temperature substrate holder within the controlled-atmosphere chamber. The substrate is usually, though not always, heated to facilitate bond making between reactive molecular species produced during a laser ablation process. During the laser ablation process, a continuous, uniform film is deposited on the surface of the substrate.
In one possible embodiment of the invention, mesoporous transition metal oxide thin films intended for use in chemical sensors may be grown to a thickness that ranges from about 10 nm to about 100 xcexcm, preferably between about 200 nm and about 300 nm. The time of deposition may range from about 30 seconds to about one hour, preferably from about 15 min to about 20 min.
In another possible embodiment of the invention, mesoporous transition metal oxide thin films intended for use in electrochromic applications may be grown to a thickness of several tens of nanometers by varying the deposition time. In this embodiment, a glass substrate is usually heated to a temperature of about 250xc2x0 C., and a MYLAR substrate is usually heated no higher than 80xc2x0 C., preferably room temperature (approximately 25xc2x0 C.) to about 30xc2x0 C. An oxygen atmosphere is preferably maintained at 200-300 mTorr. The film is grown for a time ranging from about 30 seconds to a few minutes, preferably from about 2 minutes to about 4 minutes.
A thin film produced by this method is generally poorly crystalline, as determined by x-ray diffraction (XRD) analysis. Depending on the intended application of the thin film, it may be desirable to enhance or increase the crystallinity or order of the thin film using a hydrothermal treatment. A hydrothermal treatment consists of heating a thin film-coated substrate in a sol-gel mixture. The sol-gel mixture is similar or identical to that used to synthesize the mesoporous molecular sieve target material. A hydrothermal treatment of short duration results in reorganization of the thin film material, and no increase in film thickness. A longer hydrothermal treatment generates a thicker film. A short hydrothermal treatment is defined herein as one that is less than about three days in duration. A long hydrothermal treatment can last from about 3 days to about 9 days. Because the laser deposited films act as a seed layer, the thicker the laser deposited film, the quicker the molecular sieve growth. For example, a mesoporous niobium oxide thin film generated from ablating a Nb-TMS1 target can be treated in a sol gel mixture containing niobium ethoxide, dodecylamine and water for about 1 to about 3 days at a temperature of about 180xc2x0 C. Such a treatment yields a Nb-TMS1-like film. By xe2x80x9cNb-TMS1-like filmxe2x80x9d it is meant that the film exhibits the X-ray powder diffraction patterns that would be expected for Nb-TMS1 bulk material.
The templating agent may be removed from select transition metal oxide films using an acid wash. Templating agent removal can be achieved by either calcination or washing with an acid solution. In the case of an Nb-TMS1 -like film, typical template removal conditions include immersion of the thin film in 3:1 isopropanol:water acidified using HNO3 to pH 1.75, stirred for approximately 3 hours. An acid wash can be shorter or longer, depending upon the stability of the supporting substrate at low pH values.